Despite the fact that obesity in older adults significantly increases fall risk, and the likelihood of injury and disability thereafter, an explanation for the increased risk does not exist. This limits the development of obesity-specific fall-prevention interventions. There is a critical need to identify the extent to which obesity affects modifiable mechanisms that explain the causes of falls by obese older adults. Our long-term goal is to decrease falls and their sequel by identifying biomechanical factors that increase fall risk in a population and then developing novel interventions to improve these factors. The overall objective of the proposed study is to provide an initial biomechanical explanation for fall by obese older adults by quantifying the effects of obesity on gait and on kinematics of the recovery response following postural disturbances in the laboratory, and revealing the causes of prospective falls by obese older adults. Based on strong preliminary data, our central hypothesis is that the increased fall-risk of obese older adults can be explained by the effects of obesity on modifiable biomechanical factors that increase the likelihood of a trip- related fall. This include factors related to the initiation of a trip-related fall during gait - e.g., insufficient toe clearnce which can cause a stumble - and factors related to failed recovery thereafter. Our rationale for the proposed studies is that they will provide a framework for the development of interventions to reduce obesity-related falls, which could include aspects of existing trip-related fall preventin interventions. The central hypothesis will be tested by pursuing three specific aims: 1) Quantify the effects of obesity on factors related to stumbles by older adults; 2) Quantify the effects of obesity on the mechanics of recovery stepping responses following large postural disturbances in the laboratory; and 3) Elucidate the causes for prospective, community-based falls by obese older adults and their relationship to laboratory measures. In Aim 1 the vertical trajectory of the foot, relative to a treadmill surface will be tracked during gait using standard motion capture. In Aim 2 established laboratory-induced trip and treadmill-based postural disturbance protocols will be employed and, following these disturbances, kinematics of the recovery step, center of mass and trunk will be tracked. Aim 3 will use a previously established protocol where, twice- monthly for one year, subjects respond to electronic surveys enquiring about the occurrence and cause of community-based falls. Kinematics and trip-related fall rates will be compared between obese and normal weight older adults. The contribution of this work will be significant as it represents the first step in a continuum of research expected to prevent obesity-related falls and reduce their associated costs and negative impact on well-being. The proposed study is innovative in that it explains obesity-related fall risk in terms of modifiable biomechanical, rather than body weight-related measures, thus providing a framework for treatment that is independent from weight loss.